tests: check for loopback module on pivot_root test

mount -o loop fails when the loopback module is not loaded with
permission denied. Add a check if /dev/loop0 exists. If not, load
the loop module.

MR: https://gitlab.com/apparmor/apparmor/-/merge_requests/781
Signed-off-by: Georgia Garcia <georgia.garcia@canonical.com>
Acked-by: Christian Boltz <apparmor@cboltz.de>
(cherry picked from commit 656a48b900)
Signed-off-by: John Johansen <john.johansen@canonical.com>

tests/regression/apparmor/pivot_root.sh

@@ -45,6 +45,11 @@ pivot_root_cleanup() {
 }
 do_onexit="pivot_root_cleanup"
 
+# loopback module must be loaded for mount -o loop to work
+if [ ! -b /dev/loop0 ] ; then
+	modprobe loop
+fi
+
 # systemd mounts / and everything under it MS_SHARED. This breaks
 # pivot_root entirely, so attempt to detect it, and remount /
 # MS_PRIVATE temporarily.